1. Field of the Invention
The present invention relates to a method, system, and program for establishing and maintaining a point-in-time copy.
2. Description of the Related Art
Computing systems often include one or more host computers (“hosts”) for processing data and running application programs, direct access storage devices (DASDs) for storing data, and a storage controller for controlling the transfer of data between the hosts and the DASD. Storage controllers, also referred to as control units or storage directors, manage access to a storage space comprised of numerous hard disk drives connected in a loop architecture, otherwise referred to as a Direct Access Storage Device (DASD). Hosts may communicate Input/Output (I/O) requests to the storage space through the storage controller.
In many systems, data on one storage device, such as a DASD, may be copied to the same or another storage device so that access to data volumes can be provided from two different devices. A point-in-time copy involves physically copying all the data from source volumes to target volumes so that the target volume has a copy of the data as of a point-in-time. A point-in-time copy can also be made by logically making a copy of the data and then only copying data over when necessary, in effect deferring the physical copying. This logical copy operation is performed to minimize the time during which the target and source volumes are inaccessible.
One such logical copy operation is known as FlashCopy® (FlashCopy is a registered trademark of International Business Machines, Corp. or “IBM”). FlashCopy® involves establishing a logical point-in-time relationship between source and target volumes on different devices. Once the logical relationship is established, hosts may then have immediate access to data on the source and target volumes, and the data may be copied as part of a background operation. Reads to any tracks in the target cache that have not been updated with the data from the source causes the source track to be staged to the target cache before access is provided to the track from the target cache. Any reads of data on target tracks that have not been copied over cause the data to be copied over from the source device to the target cache so that the target has the copy from the source that existed at the point-in-time of the FlashCopy® operation. Further, any writes to tracks on the source device that have not been copied over cause the tracks on the source device to be copied to the target device.
In the prior art, as part of the establishment of the logical point-in-time relationship during the FlashCopy® operation, all tracks in the source cache that are included in the FlashCopy® must be destaged to the physical source volume, e.g., source DASD, and all tracks in the target cache included in the FlashCopy® must be discarded. These destage and discard operations during the establishment of the logical copy relationship can take several seconds, during which I/O requests to the tracks involved in the copy relationship are suspended. In critical operating environments, there is a continued effort to minimize any time during which I/O access is suspended. Further details of the FlashCopy® operations are described in the copending and commonly assigned U.S. Pat. No. 6,611,901, filed on Jul. 2, 1999 and granted on Aug. 26, 2003, entitled “Method, System, and Program for Maintaining Electronic Data as of a Point-in-Time”, which patent application is incorporated herein by reference in its entirety.
For these reasons, there is a continued need in the art to reduce the time needed to complete establishing a logical point-in-time copy between a source and target volumes.